Field
Embodiments of the present invention generally relate to meteorology and weather detection, and more specifically, to detecting clouds using polarized sunlight.
Description of Related Art
Super-thin cirrus clouds with optical depths smaller than approximately 0.3 exist globally. These clouds are important to the radiation energy balance of the Earth. They also can affect the remote sensing of aerosols, surface temperature, and atmospheric gases. For example, the aerosol optical depth (AOD) from NASA's Moderate Resolution Imaging Spectroradiometer (MODIS) data could be overestimated by about 100% when these clouds exist. Failing to detect these clouds, the sea surface temperature (SST) retrieved from NASA's Atmospheric Infrared Sounder (AIRS) satellite data is about 5K lower at tropical and mid-latitude regions, where occurrence frequency of these clouds is high. Climate models must incorporate these clouds correctly in order to account for the Earth's radiation energy budget. To inter-calibrate other satellite sensors with NASA's future Climate Absolute Radiance and Refractivity Observatory (CLARREO) measurements, knowledge of these clouds is necessary for correcting these sensors' measurement errors due to light's polarization.
Due to uncertainties in surface reflectance, transparent super-thin clouds generally cannot be detected by satellite imagers, like the MODIS and the Advanced Very High Resolution Radiometer that only measure the total radiance of the reflected solar light. The resulting data products of many satellite and ground measurements are biased by these undetected clouds. Using a strong water vapor absorption channel such as the 1.38 μm radiance to exclude the surface and low-layer effects can be effective on high cirrus, but may encounter difficulties for atmospheres with low water vapor. The reliability of this method is also questionable if the clouds' optical depth is smaller than about 0.5, when their backscattered intensity is low. In addition, super-thin clouds may also exist in the lower layers of the atmosphere where there is ample water vapor. The sensitivity of the 1.38 μm channel is weak in this region, hampering detection capabilities.
NOAA's polar orbiting High Resolution Infrared Radiation Sounder multispectral infrared data are usually used with the CO2-slicing method for detecting thin cirrus clouds. However, for super-thin clouds, this requires the radiance of their background atmosphere and surface to be very close to that of the reference clear sky environment, which can be difficult as the terrestrial background changes on spatial and temporal scales. In addition, this method is problematic when the difference between clear-sky and cloudy radiance for a spectral band is less than the instrument noise, as for super-thin clouds.
Currently, NASA's Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite is the only instrument in orbit that can detect super-thin clouds. The lidar fires a laser through the atmosphere and detects the signal returning. The time and strength of the returning signal is analyzed to determine where in the atmosphere particles are located and how many there are. While this instrument is effective, it is extremely expensive to operate and can only measure a small region (i.e., due to the narrow thickness of the laser beam).
Improvements in cloud detection would be useful, as they would improve weather predictions and calculations of the energy budget.